This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-274001, filed Sep. 28, 1999; and No. 11-277454, filed Sep. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor device, and more particularly to a high voltage lateral bipolar transistor which is suitable for the incorporation into an integrated circuit.
Conventionally, a lateral bipolar transistor is formed on an SOI (silicon on insulator) so that a high voltage semiconductor device is formed. FIGS. 1A and 1B are a plan view and a sectional view showing such conventional lateral bipolar transistors.
As shown in FIGS. 1A and 1B, a buried oxide layer 52 is formed on a semiconductor substrate 51 on which a high-resistance n-type active layer 53 is formed. On a surface of the n-type active layer 53, an n-type diffusion layer 56 is formed. On the surface of this n-type diffusion layer 56, a low-resistance base contact layer 58, a p-type collector layer 59 and a p-type emitter layer 57 are selectively formed respectively. A base electrode 62b is formed on the n-type base contact layer 58, a collector electrode 62c is formed on the p-type collector layer 59, and an emitter electrode 62a is formed on the p-type emitter layer 57, respectively.
Incidentally, reference numeral 60a denotes a LOCOS (local oxidation of silicon) film. Reference numeral 60b denotes a thermal oxide film. On these oxide films, an insulation film 61 is formed. On the n-type active layer 53, a trench 53a is formed. In this trench 53a, a polycrystal silicon layer 55 is buried via the insulation layer 54 so that an isolation region is formed.
With the lateral bipolar transistor having such a structure, an impurity distribution of the n-type diffusion layer 56 forms a profile so that the concentration is high on the surface thereof, and the concentration becomes low as the depth becomes deeper. As a consequence, the current density injected from the p-type emitter layer 57 becomes high at the portion deeper than the surface portion.
Consequently, in the case where the potential of the semiconductor substrate 51 is changed with respect to the n-type active layer 53, for example, when the potential of the semiconductor substrate 51 becomes a negative value with respect to the n-type active layer 53, there arises a problem that a p-type inversion layer is formed on an interface between the buried oxide film 52 and the n-type active layer 53, and a resistance of the current channel of the hole current is remarkably lowered with the result that the characteristic such as hFE or the collector current dependency on the voltage between the emitter and the collector depends on the potential of the semiconductor substrate 51.
By the way, in order to improve the breakdown voltage of the conventional PNP-type lateral bipolar transistor, a depletion layer is widened to the base side generally by lowering an impurity concentration in the base region to reduce an electric field. However, the following problem is generated when the impurity concentration in the base region is lowered. That is, there arises a problem that the base width modulation effect is conspicuously generated wherein the base width is gradually decreased with an electric field from the collector side so that the gain is changed, with the result that the Early voltage is lowered. For example, the deterioration in the circuit performance occurs when an analog circuit is formed of transistors having a low Early voltage. As one example, there can be given a fact that the gain is lowered when a transistor which has a low Early voltage is used in a comparator. Incidentally, the Early voltage will be described later, but the Early voltage is a voltage value at a point where an abscissa intersects a straight line extended from the straight line of the collector current in the active region when the voltage between the collector and emitter is assigned to an abscissa and a collector current is assigned to an ordinate of a graph.
A first object of the present invention is to provide a structure of a semiconductor device wherein the characteristic such as hFE or the like does not depend on the potential of the semiconductor substrate.
A second object of the present invention is to provide a lateral bipolar transistor having a high Early voltage wherein the base width modulation effect is suppressed while a breakdown voltage is maintained.
In order to attain the above object, the semiconductor device according to a first aspect of the invention comprises:
a substrate a surface of which is formed of an insulation region;
a high resistance active layer of a first conductivity type formed on the surface of the substrate;
a semiconductor region of the first conductivity type having an impurity concentration higher than that of the active layer of the first conductivity type and selectively formed on a surface of the active layer of the first conductivity type;
an emitter region of a second conductivity type selectively formed on the semiconductor region of the first conductivity type;
a collector region of the second conductivity type selectively formed on the surface of the active layer of the first conductivity type in separation from the semiconductor region of the first conductivity type; and
a base contact region of the first conductivity type selectively formed on the surface of the active layer of the first conductivity type in separation from the emitter region and the collector region.
The semiconductor device according to a second aspect of the present invention comprises:
a substrate a surface of which is formed of an insulation region;
a high resistance active layer of a first conductivity type formed on the surface of the substrate;
a first semiconductor region of the first conductivity type having an impurity concentration higher than that of the active layer of the first conductivity type and selectively formed on a surface of the active layer of the first conductivity type;
an emitter region of a second conductivity type selectively formed on the surface of the first semiconductor region of the first conductivity type;
a collector region of the second conductivity type selectively formed on the surface of the active layer of the first conductivity type in separation from the first semiconductor region of the first conductivity type;
a second semiconductor region of the first conductivity type having an impurity concentration higher than of the active layer of the first conductivity type, and formed on the active layer of the first conductivity type in separation from the semiconductor region of the first conductivity type; and
a base contact region of the first conductivity type selectively formed on the surface of the second semiconductor region of the first conductivity type.
Furthermore, according to a third aspect of the present invention, the semiconductor device comprises:
a substrate a surface of which is formed of an insulation region;
a high resistance active layer of a first conductivity type formed on the surface of the substrate;
a semiconductor region of the first conductivity type having an impurity concentration higher than that of the active layer of the first conductivity type and selectively formed on a surface of the active layer of the first conductivity type;
an emitter region of a second conductivity type and a base contact region of a first conductivity type formed in separation from each other on the surface of the active layer of the first conductivity type; and
a collector region of the second conductivity type selectively formed on the surface of the active layer of the first conductivity type in separation from the semiconductor region of the first conductivity type.
It is desirable that the semiconductor device according to the first to third aspects of the present invention is constituted in the following manner.
(1) The first semiconductor region of the first conductivity type and the second semiconductor region of the first conductivity type are constituted substantially in the same depth, and substantially in the same surface impurity concentration.
(2) The depth of the semiconductor region of the first conductivity type is smaller than the thickness of the active layer of the first conductivity type.
(3) The depth of the semiconductor region of the first conductivity type is 1 xcexcm or more.
(4) The surface impurity concentration in the semiconductor region of the first conductivity type is 5xc3x971015 cmxe2x88x923 or more and 5xc3x971016 cmxe2x88x923 or less.
(5) The thickness of the active layer of the first conductivity type is 10 xcexcm or less.
(6) The breakdown voltage between the emitter region and the collector region is 10V or more and 80V or less.
(7) The emitter region is surrounded by the collector region excluding a portion between the base contact region and the emitter region.
According to the present invention, even in the case where an inversion layer is formed due to a voltage of the substrate on an interface surface of the active layer of the first conductivity type which contacts the insulation region, an emitter current which flows via the inversion layer is suppressed by the semiconductor region of the first conductivity type formed under the emitter region. As a consequence, the emitter current is allowed to flow on the surface side of the active layer of the first conductivity type. Therefore, the resistance of this current channel can be prevented from remarkably lowering, which results from the flow of the hole current via the inversion layer, with the result that it becomes possible to suppress the problem that the characteristic of hFE or the like depends on the potential of the semiconductor substrate.
According to a fourth aspect of the present invention, the semiconductor device comprises:
a semiconductor region of a first conductivity type formed on a substrate;
an emitter region of a second conductivity type and a collector region of the second conductivity type selectively formed on a surface of a semiconductor region;
a base contact region of the first conductivity type selectively formed on the surface of the semiconductor region of the first conductivity type in separation from the emitter region of the second conductivity type and the collector region of the second conductivity type; and
a first semiconductor region of the second conductivity type having an impurity concentration lower than that of the collector region of the second conductivity type, and formed between the collector region of the second conductivity type and the emitter region of the second conductivity type in contact with the collector region of the second conductivity type in separation from the emitter region of the second conductivity type.
According to a fifth aspect of the present invention, the semiconductor device comprises:
a semiconductor region of a first conductivity type formed on a substrate;
an emitter region of a second conductivity type and a collector region of the second conductivity type selectively formed on a surface of the semiconductor region;
a base contact region of the first conductivity type selectively formed on a surface of the semiconductor region of the first conductivity type in separation from the emitter region of the second conductivity type and the collector region of the second conductivity type; and
a first semiconductor region of a second conductivity type having an impurity concentration lower than that of the collector region of the second conductivity type, and formed in contact with a portion of the collector region of the second conductivity type located opposite to the emitter region of the second conductivity type and in separation from the emitter region of the second conductivity type.
It is desirable that the semiconductor device according to the fourth and the fifth aspects of the present invention is constituted in the following manner.
(1) The first semiconductor region of the second conductivity type is formed also on a portion located opposite to the base contact region of the first conductivity type in contact with the collector region of the second conductivity type, and this first semiconductor region of the second conductivity type is separated from the base contact region of the first conductivity type.
(2) The second semiconductor region of the second conductivity type which has an impurity concentration lower than that of the emitter region of the second conductivity type is provided such that the region contacts a portion opposite to the collector region of the second conductivity type in the emitter region of the second conductivity type and is formed in separation from the collector region of the second conductivity type.
(3) The second semiconductor region of the second conductivity type is also formed on a portion located opposite to the base contact region of the first conductivity type in the emitter region of the second conductivity type, and the second semiconductor region of the second conductivity type is separated from the base contact region of the first conductivity type.
(4) There are provided a source region of a second conductivity type and a drain region of the second conductivity type selectively formed in separation from each other on a surface of the active region of the first conductivity type provided on the same substrate on which the semiconductor region of the first conductivity type is formed; a third semiconductor region of the second conductivity type having a concentration lower than the concentration of the drain region of the second conductivity type, the region being formed in separation from the source region of the second conductivity type and in contact with the drain region of the second conductivity type and including a portion between the source region of the second conductivity type and the drain region of the second conductivity type, and a gate electrode formed via a gate insulation film on the surface of the active layer of the first conductivity type sandwiched between the third semiconductor region of the second conductivity type and the source region of the second conductivity type.
(5) A collector region of the second conductivity type is formed so as to surround the emitter region of the second conductivity type excluding a portion between the base contact region of the first conductivity type and the emitter region of the second conductivity type.
(6) The semiconductor region of the first conductivity type is formed on the substrate via the insulation region.
(7) The net dose of the second semiconductor region of the second conductivity type is 4.0xc3x971012 cmxe2x88x922 or less.
(8) The net dose of the first semiconductor region of the second conductivity type is 4.0xc3x971012 cmxe2x88x922 or less.
(9) The net dose of the third semiconductor region of the second conductivity type is 4.0xc3x971012 cmxe2x88x922 or less.
According to the present invention, in a lateral bipolar transistor (for example, PNP-type transistor), the depletion layer is expanded toward the collector side to maintain the breakdown voltage instead of being expanded toward the base side like the conventional transistor, thereby reducing the electric field. For this purpose, a semiconductor region of a second conductivity type is formed, which has a concentration lower than the concentration of the collector region of the second conductivity type. Within such a semiconductor region of the second conductivity type, the depletion layer is expanded from an interface with the base region, so that breakdown voltage can be secured in this semiconductor region of the second conductivity type. Consequently, since the impurity concentration of the base region can be increased, the expansion of the depletion layer in the base region can be suppressed with the result that the base width modulation effect resulting from the decrease in the base width can be suppressed and the Early voltage can be increased.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.